1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and specifically to a plasma discharge radiation source for use therein.
2. Description of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet, also referred to as XUV or EUV, radiation. The abbreviation xe2x80x98XUVxe2x80x99 generally refers to the wavelength range from several tenths of nm to several tens of nanometers, combining the soft x-ray and Vacuum UV range, whereas the term xe2x80x98EUVxe2x80x99 is normally used in conjunction with lithography (EUVL) and refers to a radiation band from approximately 5 to 20 nm, i.e. part of the XUV range.
A radiation source for XUV radiation may be a discharge plasma radiation source, in which a plasma is generated by a discharge in a vapor between an anode and a cathode and in which a high temperature discharge plasma may be created by Ohmic (resistive) heating by a (pulsed) current flowing through the plasma. As used herein, xe2x80x9cvaporxe2x80x9d is intended broadly to include a gas, a suspension or a mixture. Further, compression of a plasma having some volume due to a magnetic field generated by a current flowing through the plasma may be used to create a high temperature, high density plasma on a discharge axis (dynamical pinch effect). Kinetic energy of the pinching plasma is directly transferred to the plasma temperature and hence to short-wavelength radiation. A dynamical pinch would allow for a discharge plasma having a considerably higher temperature and density on the discharge axis, offering an extremely large conversion efficiency of stored electrical energy into thermal plasma energy and thus into XUV radiation.
It has been proposed by R. Lebert, K. Bergmann, G. Schriever and W. Neff in a presentation entitled xe2x80x98A gas discharge based radiation source for EUVLxe2x80x99, Sematech Workshop Monterey (1999), to employ a hollow cathode for triggering plasma creation. A very effective way of self-initiation of a discharge may be obtained by a so-called transient hollow cathode discharge (THCD) in the hollow cathode. The radiation source as proposed by Lebert et al. is an axi-symmetric system with a specially configured cathode having a small aperture on-axis with a large cavity behind it forming a hollow cathode region. However, the hollow cathode generates the discharge breakdown and thus a plasma only on or in a small volume around the discharge axis, which does not allow making (full) use of the dynamical pinch effect referred to above. Further, the volume taken by a plasma around the discharge or central axis is generally badly controllable and its stagnation on the discharge axis after compression is therefore not sufficiently predictable to have an exactly timed pulse of generated XUV radiation.
Another drawback of a plasma discharge radiation source having the conventional central hollow cathode is that the plasma created may erode and change the form of the aperture of the hollow cathode, since it is present on the axis on which the high temperature plasma having a considerable density is created. The aperture will therefore be damaged by unavoidable axially oriented plasma jets, which limits the lifetime of the cathode and decreases the maintenance interval of the radiation source. Further, proper functioning of the hollow cathode for triggering the plasma is dependent on a predetermined relation between size of the aperture and depth of the cavity. Erosion of the aperture therefore undesirably influences the triggering instant of the plasma and the timing of the pulse of generated XUV radiation.
In one aspect of the present invention a radiation source is provided having an effective self-initiation that allows making use of the dynamical pinch effect to create a high-temperature, high-density plasma for an enhanced conversion efficiency of electrical energy into radiation.
Another aspect the invention provides a radiation source having a long maintenance interval.
Another aspect of the invention provides a radiation source having a well-defined timing of generated pulses, or shots, of XUV radiation.
According to one aspect of the present invention there is provided a radiation source comprising an anode and a cathode that are configured and arranged to create a discharge in a vapor in a space between said anode and cathode and to form a plasma of a working vapor so as to generate electromagnetic radiation. The cathode further defines a hollow cavity in communication with the discharge region through an aperture that has a substantially annular configuration around a central axis of said radiation source so as to initiate said discharge. The working vapor may include, for example, xenon, lithium vapor or tin vapor.
The present invention can provide that the discharge is being initiated by the annular hollow cathode at a predetermined distance from a central axis of the radiation source and take advantage of the dynamical pinch effect. The discharge is created at least at a distance from the central axis corresponding to the annular aperture so as to create an initial plasma. An electrical current flowing through the plasma between anode and cathode generates a magnetic field compressing the plasma from the distance (or radius) corresponding to the annular aperture towards the central axis so as to create a dense and hot plasma.
The discharge plasma created above the annular aperture may be chosen to have a low density such as not to cause erosion of the aperture. Both the density of the plasma and its distance to the annular aperture will increase upon compression towards the central axis. The distance of annular aperture may be chosen large enough not to cause erosion of the aperture at the final stagnation, or collapse, of the plasma on the central axis.
Further, the predetermined distance between annular hollow cathode provides a control radius where plasma compression starts, which results in a well-controllable timing of its collapse and generation of a pulse of XUV radiation.
In certain embodiments a driver vapor is supplied to said cavity. Further, the working vapor may be conveniently supplied in a region around said central axis of said space between said anode and cathode. In such embodiments control over the generation of pulses of XUV radiation is improved.
The hollow cathode provides for an effective self-organized discharge initiation having the practical feature that high repetition rates along with a high reproducibility may be achieved. The operation can basically be auto-triggered. To further increase the exact timing of the discharge and finally of the generation of a pulse of XUV radiation, one embodiment of the radiation source according to the invention includes a trigger electrode that is inserted in the cavity. When the radiation source is in a state that auto-triggering is almost going to take place, a voltage pulse applied to the trigger electrode will cause such a disturbance of the electrical field inside the cavity that the process will trigger. Any uncertainty in the timing of auto-triggering will in this way be eliminated by exactly timing a trigger pulse being applied to the trigger electrode. Appropriate electrical circuitry is employed to apply a voltage pulse to said trigger electrode.
To provide for a stable shot-to-shot electrically stored energy and a very fast supply of electrical energy to anode and cathode, the radiation source may include a capacitor connected to said anode and cathode, and may further include a charging circuit connected to said capacitor, said charging circuit including a further capacitor and a transformer for electrically coupling said capacitor and said further capacitor. Switches may control charging of said capacitor by said further capacitor, and of said further capacitor by a source of electrical power.
An electrical insulator will be provided to separate anode and cathode. To prevent degradation of the insulator by lithium vapor or tin vapor, a path between a region of said space between said anode and cathode where said vapor is provided and said electrical insulator is constructed and arranged to define a space for said vapor to condense along said path.
According to a further aspect of the present invention there is provided a lithographic projection apparatus including a radiation system for providing a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate; and a projection system for projecting the patterned beam onto a target portion of the substrate, wherein said radiation system includes a radiation source as described above.
The present invention also provides a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material, providing a projection beam of radiation using a radiation system comprising a radiation source as described above, using patterning structure to endow the projection beam with a pattern in its cross-section, and projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultraviolet radiation (XUV or EUV, e.g. having a wavelength in the range of 5 to 20 nm).